Control for roll gap of a rolling mill

ABSTRACT

A feedback control system for a gap between rolls in a mill includes a variable inductance with a core on one roll and an armature on the other roll. This inductance, as well as a reference inductance and two auxiliary inductances, are interconnected to establish AC biased bridge. The relative phase of the bridge diagonal voltage is used to control the gap width. The pick up inductance and one of the auxiliary inductance are mounted in physical proximity to each other.

United States Patent Girlatschek 51 May 16, 1972 [54] CONTROL FOR ROLLGAP OF A ROLLING MILL [72] Inventor: Siegfried Girlatschek, Bremen,Germany [73] Assignee: Vereinigte Flugtechnische Werke Fokker GmbH,Bremen, Germany [22] Filed: Apr. 13, 1970 [2]] Appl. No.: 28,351

[30] Foreign Application Priority Data June 21, 1969 Germany ..P I9 31654.2

[52] US. Cl. ..72/21, 324/34 TX 51 1 men ..B21b37/08 [58] Fieldoi'Search..72/7,8,2l;324/34TK,37

[56] References Cited UNITED STATES PATENTS 3,516,273 6/1970 Stone..72/21 2,934,698 4/1960 Longland ..324/41 3 ,441 ,840 4/ l 969 Randle324/34 TK 2,711,510 6/l955 Tricebock ..324/41 3,208,251 9/l965 Hulls etal ..72/l 1 3,492,845 2/ l 970 Nomura ..72/8 3,389,588 6/1968 Reinhardtet al. .....72/8

Primary Examiner-Milton S. Mehr Attorney-Smyth, Roston and Pavitt andRalf Siegemund [57] ABSTRACT A feedback control system for a gap betweenrolls in a mill includes a variable inductance with a core on one rolland an ar mature on the other roll. This inductance, as well as areference inductance and two auxiliary inductances, are interconnectedto establish AC biased bridge. The relative phase of the bridge diagonalvoltage is used to control the gap width. The pick up inductance and oneof the auxiliary inductance are mounted in physical proximity to eachother.

10 Claims, 3 Drawing Figures PATENTEB m 16 I972 SHEET 1 [IF 2 Fig. 2

Inventor.-

PATENTEDMM 16 1972 SHEET 2 OF 2 Fig.3

CONTROL FOR ROLL GAP OF A ROLLING MILL The present invention relates toan arrangement and apparatus for controlling the gap or clearance of andbetween the rolls of a pair of rolls in a rolling mill, to operate bymeans of automatic control equipment and using particularly inductancemeans for measuring the width of the gap. The inductance means is toinclude a core and an armature, and the effective inductivity of thatinductance is variable through variation of the air gap between core andarmature.

It is customary in rolling mills to adjust the desired thickness of theproduct to be rolled by adjusting the pressure exerted upon and by theoperating rolls. Such a more or less indirect control does not permitcompensation for all kinds of disturbances as they may arise.Accordingly, the rolled stock has accurate thickness only within afairly large range of tolerances. In order to narrow the tolerances itis necessary to provide direct control which includes rather exactmeasuring of the roll gap. Such a requirement for obtaining sufiicientlyaccurate control is best carried out by means of electronic control andregulating equipment.

For purposes of gap control through a feedback system it is, of course,necessary to measure the gap between the rolls and to providerepresentation thereof as an electrical quantity. The roll gap width ofa pair of rolls has been detected previously directly and electronicallyby means of optical scanners or by means of feeler and lever systemscoupled to a potentiometer or to an adjustable inductance. It was found,however, that particularly the latter kind of electromechanicalequipment is rather complicated, and adjustment thereof is ratherdifficult to obtain. Cumbersome adjustment procedure is particularlydisadvantageous for cold rolling because for reasons of surface finishit is required here to change the operating rolls about every 2 to 3hours.

Another known method for measuring the roll gap is ascertained bycontactless gauging. This method utilizes a stationary magnetic fieldfor measuring the roll gap. The strength of the magnetic field isapproximately inversely proportional to the roll gap operating as an airgap within the magnetic field path. The dimensions of this air gap aremeasured by placing Hall generators or indium antimony probes (fieldplates) into the magnetic field path. The electrical output derivablefrom these pick up and field sensing element includes indeedrepresentation of the roll gap as an electrical signal.

It was found that the aforementioned method requires rather strongmagnetic fields across this roll air gap because field plates and Hallgenerators are rather insensitive to weak magnetic fields. In case astrong magnetic field is employed, inaccuracies are introduced in themeasuring result particularly if the rolled stock is ferromagneticmaterial. In this case shavings and other particles may be attracted bythe rolls, and it is difficult to protect the measuring instrumentagainst the resulting disturbances on the probing field. Furthermore,the production of a stationary and constant magnetic field requires thatthe field generating current has to be maintained constant ratheraccurately, as fluctuations in the exciter current deteriorate theaccuracy of measuring.

Another point to be considered is that production of a sufficientlystrong magnetic field is accompanied by considerable heating of theinstrument. This fact, in turn, requires additional provisions fortemperature compensation. It can be seen that employment of a strong,constant magnetic field requires considerable expenditure and has manyinherent disadvantages; it is, therefore, not surprising that thismethod has not gained acceptance.

The problem, therefore, still exists to provide a method for measuringthe gap width between the rolls of a pair of rolls in a rolling mill,and seemingly utilization of capacitive or inductive phenomena may besuited best. However, the gap detector cannot be protected againstcooling liquid during rolling. Thus, capacitive methods must be excludedfor all practical purposes because the coolants have relatively largedielectric constants. This narrows the suitable apparatus to employmentand utilization of inductive phenomena. Considering this point, theinvention relies on particular utilization of inductive phenomena as wasoutlined in the introduction but obviating the prior art deficiencies.

In accordance with the preferred embodiment of the present invention, itis suggested that the gap width be represented by variation of an airgap between a core of a pick up inductance and an armature thereof.Armature and core are disposed in two separate carriers whichrespectively receive journals. of the rolls, so that the distancebetween core and armature represents the roll gap as air gap in theinductance. The core of this pick up inductance as well as an auxiliaryinductance are mounted together in one of the two carriers. These twoinductances are included in a bridge as two serial branches connectedacross a bridgebiasing source. The bridge includes additionally anadjustable inductance to provide a desired or reference value, andanother auxiliary inductance completes the bridge. A control signal isderived from one of the diagonals of the bridge. In case the actualvalue of the roll gap deviates from the desired value, that controlsignal is used to operate upon a controller for adjusting the roll gapand completing a feedback loop.

The inventive arrangement is characterized particularly by a rathersimple construction, by insensitivity against mechanical disturbances,and particularly by ready adaption to large and small gaps. Moreover,the resolution of the roll gap measuring method and accuracy thereof areparticularly high. Installation of the equipment does not posedifficulties particularly because a reference value of zero for the gapwidth to be measured can be established simply by moving the armatureinto engagement with the core corresponding to a gap width zero, and theresulting inductivity defines the base line or zero reference for gapmeasurement.

The combination of pick up inductance and of one of the auxiliaryinductances of the bridge provides a pick up device from which, withincertain limits, influences of temperature have been eliminated, becauseboth inductances as well as their loss resistances are subjected to thesame thermal conditions. Another advantage results from a rather simplelayout of the entire electronic circuitry, because voltage and currentstabilization is no longer required. Moreover, it is no longer necessaryto provide particular linearization as far as relationship of desiredand actual value for the gap is concerned. Furthermore, the dynamics ofthe control loop is independent from the adjusted air gap of the pick upinductance.

The arrangement in accordance with the invention is also useful inrolling mills used for rolling ferromagnetic material because the low ACinduction in the gap area is insufficient to attract any shavings orcuttings. Finally, the inductive phenomena utilized is not sensitive toengagement with a coolant.

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter which is regarded as theinvention, it is believed that the invention, the objects and featuresof the invention, andfurther objects, features and advantages thereofwill be better understood from the following description taken inconnection with the accompanying drawings, in which:

FIG. 1 illustrates somewhat schematically the basic arrangement of aroll gap control apparatus improved in accordance with the presentinvention;

FIG. 2 illustrates the measuring and pick-up instrumentality employed inthe arrangement of FIG. 1; and

FIG. 3 illustrates somewhat schematically a circuit diagram of theentire roll gap control loop.

Proceeding now to the detailed description of the drawings, FIG. 1illustrates, as stated, somewhat schematically the basic arrangement forroll gap control. A journal 11 extends axially from a lower roll 10 andcarries a support ring 13 for a pick-up means carrier 12. The rollingmill includes an upper roll 14, and the material 16 to be rolled, i.e.,the stock is disposed in the gap 19 between rolls l0 and 14. The upperroll 14 has likewise a journal 15 carrying a ring 18 upon which isplaced a second carrier 17 constituting measuring plate.

The measuring instrumentation and pick-up means are included in carrier12 as will be described more fully below. Suffice it to say presentlythat an output cable means 12a provides electrical signal whichrepresents the width of roll gap 19. A control circuit 20 receives inaddition a reference signal representing the desired value for the gapand being developed by an appropriate signal source 21. The controlcircuit 20, of course, includes amplifier means and provides an outputused to control an electrohydraulical servo valve 22. Valve 22 governsan hydraulic actuation 23 which detennines and adjusts the pressureexerted upon rolls l and 14 for adjustment of the roll gap.

The pick-up device 12 is shown in greater details in FIG. 2 andparticularly its cooperation with the measuring plate 17 is shown withparticularity. The pick-up means 12 includes a pair of coils 30connected in series and being wound upon the two legs'33 of a U-shapedcore 32 to establish a pick-up inductance. The two legs 33 respectivelyhave front faces that are oriented coplanar to each other and transverseto the plane of the U of core 32, whereby particularly these two frontfaces of legs 33 define a datum plane 34. The second carrier 17 of thepick-up system is positioned adjacent to the datum plane 34 and definesparticularly a second measuring plane 36 that is parallel to plane 34.Carrier 17 is provided particularly for the support and positioning ofan armature 35 extending lengthwise to plane 36, and the one side ofarmature 35 facing the legs 33 of core 32 is in that plane 36. Plane 36may also define the lower surface of measuring plate 17 as a whole. Thetwo planes 34 and 36 are spaced apart by a gap 37. The entire equipmentis adjusted so that distance between planes 34 and 36 corresponds to thedistance or gap width of the rolls across gap 19. Therefore, the rollgap is represented by the distance between the core 32 and armature 35,establishing a variable air gap for the pick up inductance.

The pick-up means includes further an auxiliary inductance 31 whichincludes a pair of coils connected in series and wound upon the legs ofa U-shaped core 38. The auxiliary inductance includes likewise anarmature 39, and the two legs of core 38 have front faces facingarmature 39. There is an air gap between core and armature which isadjusted to obtain a fixed spatial relationship.

The auxiliary inductance 31 is completely embedded in shielding 40 madeof material of a high magnetic permeability. Likewise, core 32 isembedded in a shield 42 of similar material but being open in plane 34.Armature 35 is lined by shielding 41 of material of high magneticpermeability but the shield is open in plane 36. Thus, the air gapbetween armature 35 and core 32 is not lined with shielding.

As is shown also schematically in FIG. 2, the inductance as establishedby two serially interconnected coils 30 on legs 33 is connected inseries with the auxiliary inductance 31'. The common junction providesand is connected to a measuring outlet line 26. The respective two otherends of inductances 30 and 31 lead to two additional connecting lines 25and 27, provided for applying voltage across the two serially connectedinductance systems. Accordingly, there are three connecting lines forthis particular pick-up arrangement.

FIG. 3 illustrates a circuit in which the several coils appearasinductances 30 and 31 and wherein particularly inductance 30represents the principal source for measuring input signal. The seriescircuit connection of inductances 30 and 31 is likewise depicted in FIG.3, and it can be seen that the two inductances form two branches of abridge circuit. Two inductances 50 and 51 are connected in series toeach other,

and they are additionally connected in parallel to inductances 30 and31, for completing the bridge. Inductance 50 is variable to provide andto establish reference signal in representation of a desired value forthe roll gap. Inductance 51 is the second auxiliary induction in thesystem.

While not essential in principle it is convenient to think ofinductances 30 and 50 as being similarly constructed. Thus, inductance50 may include a core with an armature, and the armature is adjustablypositioned to vary the air gap in that inductance. For example, the airgap is adjusted by means of a micrometer screw to assume the same valuethat is desired for the roll gap. Inductances 31 and 51 may also besimilar. This similarity is appropriate to obtain complete electricsymmetry in the bridge circuit.

Inductances 50 and 51 can be regarded as representing the referencemeans, summarily denoted by numeral 21 in FIG. 1. The referenceinductance 50 and the auxiliary inductance 51 are particularly connectedto inductances 30 and 31 in that the one end of auxiliary inductance 51connects to one end of auxiliary inductance 31 to form a bridge terminalor junction A. The one end of inductance 50 connects to one end ofinductance 30 to form another bridge terminal or junction B. The twoterminal A and B are connected to a biasing source, AC generator 55. Thevoltage drop across inductance 30 can be regarded as signal representingthe actual value for the gap, while the voltage drop across inductance50 constitutesthe reference signal.

The junction between inductances 50 and 51 constitutes a third bridgeterminal D and is grounded. The bridge output signal can, therefor, betaken from the fourth bridge terminal C, with reference to ground.Terminal C is, of course, the connecting junction of inductances 30 and31.

Resistances RCU are included in the bridge circuit, merely to representohmic losses of the respective inductances. However, during operationthe bridge is presumed to remain balanced as to ohmic losses. Trimmingresistors may be included in the circuit if necessary to obtain balancebetween ohmic losses and inductance drops.

A phase detector or phase sensitive rectifier 56 has one input of afirst pair of AC input terminal connected to ground (i.e. to bridgeterminal D), while the second AC input of the first pair is connected tobridge terminal C which is actually line 26 as leading from the junctionof inductances 30 and 31. The phase dependent rectifier 56 has a secondpair of AC input terminal connected to generator 55. The output ofrectifier 56 represents the phase difference between the bridge biasvoltage and the bridge voltage as derived from across diagonal C-D, andthis phase difference, in turn, is a representation of the differencesin efiective inductance of the inductances 30 and 50.

The DC output voltage of phase dependent rectifier 56 is passed througha filter 57 to eliminate AC components and is fed to a control amplifier59. Amplifier 59 controls the servo valve 22 which, in turn, and as wasoutlined above, controls hydraulic means 23 for adjusting pressure uponthe lower roll 10. As a consequence, the gap between rolls l0 and 14 ischanged which, in turn, reflects upon the inductance 30 for closing thecontrol loop. The dashed line between elements 22 and 30 shows theactuation path of the feedback loop.

Control amplifier 59 is additionally connected to'a potentiometer 58having its tap connected to the micrometer spindle by means of whichinductance 50 is adjusted. The adjustable resistor 58 is included in theamplifier circuit to determine the gain of the amplifier. Thus, theamplifier gain is made dependent upon the adjusted reference valueforthe gap.

All four inductances of the system in the bridge circuit are rated, andare particularly constructed, so that they have. similar inductivity forsimilar air gaps. As stated above, the measuring inductance 30 and theauxiliary inductance 31 are combined to form a measuring pick-up means,and the reference inductance 50 may be disposed in relation to auxiliaryinductance 51 to form a structurally combined reference means. Due tophysical association of inductances that connect serially across thesupply source 55, the bridge is rendered independent from temperaturevariations that affect serially connected inductances similarily.Temperature variations afiect particularly loss resistances R in therespective inductances. Even if the bridge is detuned to some extentbecause of imbalance among the loss resistances, errors are notintroduced into the measuring output, because unbalanced signalcomponents across the loss resistances exhibit a phase shift by inrelation to the control information roper included in the bridge voltageacross terminals C and D. Therefor, these residual errors appear as ACsignals without DC components in the output of phase dependent rectifier56 and are eliminated by filter 57 accordingly.

If the measuring equipment .is employed, for example, in a hot rollingmill, temperature variations that the equipment may have to experiencecan be expected to be quite large. Severe thermal imbalance can becompensated by embedding resistors with negative temperaturecoefficients in the coils of the several inductances. As these resistorsare also included in the bridge circuit, temperature compensation isobtained, that is effective particularly for large scale temperaturevariations, requiring very little additional equipment. As the bridgecircuit is symmetrically constructed and connected even significanttemperature variations hardly affect accuracy of measurement.

Generator 55 supplies the bridge with AC having a frequency selected sothat WL/RCU for any of the individual inductances is quite large. On theother hand, the frequency should not be so high that eddy current lossesin the cores of the several inductances or changes in capacity of theconnecting lines exert any significant influence upon the accuracy ofmeasurement.

As can be seen, the circuit in FIG. 3 is connected and designed tocontrol amplifier 59 receives information from the bridge circuit viaphase dependent phase rectifier or phase detector 56 as so-called zeroinformation, i.e., in case the reference value for the roll gap agreeswith the actual value thereof. To obtain this static condition,additional off-set means may be included in the input circuit foramplifier 59. Amplifier 59 receives zero input. This mode of developingcontrol signals for the regulation eliminates the requirement for theusual constant current or constant voltage sources which, if needed inthe present case, would require considerable expenditure due torequirement of high signal-to-noise ratio. The high permeability shieldsof the several inductances eliminate external stay field and improve thesignal-to-noise ratio further.

The control circuit includes amplifying circuitry 59 that is operated inaccordance with the principles of operational amplifiers, in which thedesired response is obtained through adjustment by means of an externalnetwork. Additional input signals for control and regulator amplifiercircuit 59 may be combined with the output from filter 57, for example,in accordance with the principle of error feed forward operation.

As stated above, potentiometer 58 is ganged with the micrometer spindlefor adjusting the reference value in inductance 50, and thispotentiometer 58 is included in the amplifier circuit. This way the loopgain of the control and regulating loop is maintained constant and,therefore, optimizes transient response behavior of the system to beindependent from the adjusted reference value for the roll gap. Thiscompensation of loop gain is necessary as the measuring in ductancevaries hyperbolically with width of gap 37, and the voltage gradientacross the bridge decreases with increasing in the air gap.

The arrangement in accordance with the embodiment of the presentinvention as aforedescribed, may be modified in that the carrier 12 forthe pick-up means is by itself constructed of ferromagnetic material toprovide magnetic shielding directly. Analogously, the measuring plate 17can be constructed from ferromagnetic material obviating in this casethe particular shield 41.

The invention is not limited to the embodiments described above, but allchanges and modifications thereof not constituting departures from thespirit and scope of the invention are intended to be included.

lclaim:

1. Apparatus for control of the gap between a pair of rolls in a mill,the rolls of the pair having journals, comprising:

a first carrier on one of said journals;

a second carrier on the other one of the journals, the first and secondcarriers spaced apart; first means defining a pick-up inductance m thefirst carrier ductance of the first means corresponding to the gap widthbetween the rolls;

third means, including first and second reference inductance means, atleast one thereof being adjustable, bothof them being connected to theinductance of the first and second means to define a bridge circuit;

fourth means including a source of AC potential connected toelectrically energizing the bridge circuit;

fifth means connected to the bridge circuit to derive therefrom acontrol signal; and

control means connected to be operated in response to the control signalto adjust the gap of the rolls so as to complete a control loop.

2. Apparatus as in claim 1, the inductances of the first, second andthird means each having an air gap, the air gap of the auxiliaryinductance and of the first and second reference inductance means beingselected and adjusted to remain constant, the air gap of at least one ofthe reference inductance means adjustable to obtain reference valueadjustment, there being means for adjusting the air gap of the onereference inductance means, the inductances having equal inductivity forsimilarly adjusted air gaps.

3. Apparatus as in claim 2, wherein at least one of the referenceinductance means is adjustable by micrometer means.

4. Apparatus as in claim 1, the inductances of the first and secondmeans connected in series to each other and parallel to the fourthmeans, the reference inductances of the third means connected in seriesto each other and in parallel to the inductances of the first and secondmeans as connected to the fourth means, the control voltage beingderived from between the junction of the inductances of the first andsecond means and the junction of the references inductances of the thirdmeans.

5. Apparatus as in claim 4, the AC voltage having frequency so that theinductivity vs ohmic loss ratio of either inductance is relativelylarger at still low eddy current losses in the respective cores andrelatively low capacitive losses.

6. Apparatus as in claim 4, the fifths means including a phase sensitiverectifier connected to receive the AC voltage as provided by the fourthmeans and further connected to receive a bridge voltage as derived fromthe junctions to provide the control voltage, the control voltage beingzero when the roll gap agrees with the reference gap, the fifths meansincluding a filter to eliminate the a.c. component from the controlvoltage.

7. Apparatus as in claim 6, the control means including an operationalamplifier, the one reference inductance having adjusting means coupledto the operational amplifier to vary the gain thereof to maintain theloop gain of the control loop.

8. Apparatus as in claim 6, the operational amplifier having means toadjust its response.

9. Apparatus as in claim 1, the first and second carriers made ofnon-magnetic material, the inductances in the first carrier and thearmature in the second carrier being shielded by means of materialhaving high magnetic permeability.

10. Apparatus as in claim 1, the first and second carriers made offerromagnetic material to serve as magnetic shields.

1. Apparatus for control of the gap between a pair of rolls in a mill,the rolls of the pair having journals, comprising: a first carrier onone of said journals; a second carrier on the other one of the journals,the first and second carriers spaced apart; first means defining apick-up inductance in the first carrier and including a U-shaped corehaving legs establishing a datum plane, and coil means on the core;second means defining an auxiliary inductance in the first carrier anddisposed in vicinity of the pick-up inductance to be subjected tosimilar temperature; armature means in the second carrier andmagnetically cooperating with the core of the first carrier but spacedfrom the datum plane, so that the distance between the first and secondcarrier determines an air gap in the inductance of the first meanscorresponding to the gap width between the rolls; third means, includingfirst and second reference inductance means, at least one thereof beingadjustable, both of them being connected to the inductance of the firstand second means to define a bridge circuit; fourth means including asource of AC potential connected to electrically energizing the bridgecircuit; fifth means connected to the bridge circuit to derive therefroma control signal; and control means connected to be operated in responseto the control signal to adjust the gap of the rolls so as to complete acontrol loop.
 2. Apparatus as in claim 1, the inductances of the first,second and third means each having an air gap, the air gap of theauxiliary inductance and of the first and second reference inductancemeans being selected and adjusted to remain constant, the air gap of atleast one of the reference inductance means adjustable to obtainreference value adjustment, there bEing means for adjusting the air gapof the one reference inductance means, the inductances having equalinductivity for similarly adjusted air gaps.
 3. Apparatus as in claim 2,wherein at least one of the reference inductance means is adjustable bymicrometer means.
 4. Apparatus as in claim 1, the inductances of thefirst and second means connected in series to each other and parallel tothe fourth means, the reference inductances of the third means connectedin series to each other and in parallel to the inductances of the firstand second means as connected to the fourth means, the control voltagebeing derived from between the junction of the inductances of the firstand second means and the junction of the references inductances of thethird means.
 5. Apparatus as in claim 4, the AC voltage having frequencyso that the inductivity vs ohmic loss ratio of either inductance isrelatively larger at still low eddy current losses in the respectivecores and relatively low capacitive losses.
 6. Apparatus as in claim 4,the fifths means including a phase sensitive rectifier connected toreceive the AC voltage as provided by the fourth means and furtherconnected to receive a bridge voltage as derived from the junctions toprovide the control voltage, the control voltage being zero when theroll gap agrees with the reference gap, the fifths means including afilter to eliminate the a.c. component from the control voltage. 7.Apparatus as in claim 6, the control means including an operationalamplifier, the one reference inductance having adjusting means coupledto the operational amplifier to vary the gain thereof to maintain theloop gain of the control loop.
 8. Apparatus as in claim 6, theoperational amplifier having means to adjust its response.
 9. Apparatusas in claim 1, the first and second carriers made of non-magneticmaterial, the inductances in the first carrier and the armature in thesecond carrier being shielded by means of material having high magneticpermeability.
 10. Apparatus as in claim 1, the first and second carriersmade of ferromagnetic material to serve as magnetic shields.